1. Field of the Invention
The present invention concerns a semiconductor optical amplifier.
It finds one particular application when a signal carried by an optical wave in the form of amplitude modulation is to be carried by another optical wave, which constitutes transcription of this signal. A transcription device is then used and two waves are injected simultaneously into this device. A first wave is modulated by the signal to be transcribed. The device transcribes this signal onto the second wave.
A device of this kind can be used for wavelength conversion: in a frequency-division multiplex communication network a signal is received by one node of the network on a first spectral channel and is transmitted to the next node on a second spectral channel that may not be the same as the first. The second wave is unmodulated at the input of the device.
A device of the above kind can also be used for time-division demultiplexing or for all-optical clock recovery.
For applications such as these very high operating speeds are required. They typically correspond to a bandwidth in the order 10 GHz.
2. Description of the Prior Art
An economically advantageous way of producing a transcription device of the above kind known in itself uses a type III-V semiconductor amplifier operating near saturation. The amplitude modulation of the first wave modulates the charge carrier density in the amplifier. This density modulation leads to a corresponding modulation of the gain of this amplifier for the second lightwave. It therefore applies to this second wave amplitude modulation complementary to that of the first wave, which performs the required transcription.
In the case of the applications mentioned above, it has become apparent that the operating speed of the devices is not as great as would be desirable and is limited not only by the lifetime of the charge carriers in the amplifying medium but also because this medium is operating near saturation. This results in a lack of homogeneity in the distribution of the charge carriers over the length of the amplifier guide, this lack of homogeneity being linked to that of the longitudinal distribution of optical power. Two solutions have been considered to this problem of the intrinsic speed limitation relating to the carriers:
A first solution consists of using phase modulation in an interferometer device (of the MZ-SOA type) instead of gain modulation due to the depletion of carriers. This solution requires the implementation of an integrated interferometer incorporating at least two semiconductor amplifiers. It is technologically complex. It does not improve the component itself, but makes use of a different, faster physical effect. It is described in the following communication:
Penalty free All-optical wavelength conversion by SOAs in Mach-Zehnder Configuration, T. Durhuus et al., Ecoc'93, paper Tu C 5.2, pp.129-132. PA0 Enhanced recovery rates in semiconductor laser amplifiers using optical pumping, Manning et al., Elec. Lett., vol.30, no 10, pp.787-788, 1994.
A second solution is to employ very strong optical pumping of the amplification medium so that the equilibrium density of the carriers is reached faster. This method is effective but necessitates the costly use of a high-power third beam (called the "holding" beam). It is described in the article:
An aim of the present invention is to provide an optical amplifier with an increased speed of operation when used as a signal transcription device.
A more general aim of the present invention is to provide a semiconductor optical amplifier such that the longitudinal distribution of the charge carrier density and/or the longitudinal distribution of the optical power density in the amplifier optical guide is better suited than previously to the use of this amplifier, especially when the medium constituting the guide has to operate near saturation.
A further aim of the present invention is to provide an amplifier of the above kind in a simple, low-cost manner and in an integrated form.